<!DOCTYPE html>
<html>

<head>
    <title>Bootstrap 实例</title>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/4.0.0-beta/css/bootstrap.min.css">
    <script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js"></script>
    <script src="https://cdn.bootcss.com/popper.js/1.12.5/umd/popper.min.js"></script>
    <script src="https://cdn.bootcss.com/bootstrap/4.0.0-beta/js/bootstrap.min.js"></script>
    <style>
        body {
            position: relative;
        }
    </style>
</head>

<body data-spy="scroll" data-target=".navbar" data-offset="50">

    <nav class="navbar navbar-expand-sm bg-dark navbar-dark fixed-top">
        <ul class="navbar-nav">
            <li class="nav-item">
                <a class="nav-link" href="#section1">Section 1</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#section2">Section 2</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="#section3">Section 3</a>
            </li>
            <li class="nav-item dropdown">
                <a class="nav-link dropdown-toggle" href="#" id="navbardrop" data-toggle="dropdown">
                    Section 4
                </a>
                <div class="dropdown-menu">
                    <a class="dropdown-item" href="#section41">Link 1</a>
                    <a class="dropdown-item" href="#section42">Link 2</a>
                </div>
            </li>
        </ul>
    </nav>

    <div id="section1" class="container-fluid" style="padding-top:70px;padding-bottom:70px">
        <h1>Section 1</h1>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
    </div>
    <div id="section2" class="container-fluid " style="padding-top:70px;padding-bottom:70px">
        <h1>Section 2</h1>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
    </div>
    <div id="section3" class="container-fluid" style="padding-top:70px;padding-bottom:70px">
        <h1>Section 3</h1>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
    </div>
    <div id="section41" class="container-fluid " style="padding-top:70px;padding-bottom:70px">
        <h1>Section 4 Submenu 1</h1>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
    </div>
    <div id="section42" class="container-fluid bg-info" style="padding-top:70px;padding-bottom:70px">
        <h1>Section 4 Submenu 2</h1>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
        <p>Try to scroll this section and look at the navigation bar while scrolling! Try to scroll this section and look at
            the navigation bar while scrolling!</p>
    </div>

</body>

</html>